Yeah, so he bulked up for the combine because he was consuming (1) 40-60 grams of protein 6-7 times per day and (2) he trained hard.
But how aggressive was he post-combine in adhering to the discipline of that much protein intake over the day.
If you don't keep it up, you lose mass, which he likely did post-combine
And building mass during the season is difficult because drills, practices, film room study takes away from all out dedication to fitness that is possible during the offseason.